The present invention relates to the artificial illumination arts. It finds particular application in protecting transistors in lamp ballasts from reverse current flow during switching operations and will be described with particular reference thereto. It is to be appreciated, however, that the present invention is also applicable to protecting vital components of other circuits from voltage and current surges, and is not limited to the aforementioned application.
Typical lamp ballasts driven off of a direct current (DC) bus signal include a pair of transistors that convert the DC signal to an alternating current (AC) signal for driving a lamp operably connected to the ballast. A fundamental current Io, lags the driving voltage wave Vo. As a result, there are periods of time where current flows in a reverse direction over the transistors. Typically, the transistors are connected across high bus potentials, on the order of hundreds of volts, and even the smallest currents flowing backwards through the transistors cause high power dissipations. As a transistor dissipates power, it generates heat. The hotter a transistor becomes, the more inefficient it becomes, having a cumulative effect over time.
To help alleviate the reverse current flow problem, many ballast circuits incorporate diodes in anti-parallel relationships with the transistors, such as circuits similar to the one summarized in FIG. 1. The anti-parallel diodes provide low resistance paths to shunt reverse current away from the transistors. However, typical diodes do not exhibit instantaneous forward recovery characteristics, that is, the diodes do not immediately turn conductive the moment a forward bias is applied. In a time period where the forward recovery threshold of the diode has not been reached, reverse current still flows across the inactive transistor, turning both transistors active, temporarily.